Flash producing circuit



Apnl 5, 19.60 R. L. FORGACS FLASH PRODUCING CIRCUIT Filed Aug. 23, 1956 m H B H I m m m R.L. FORGACS INVENTOR.

ATTORNEYS Accordingly, it is an object of United States Patent FLASH PRODUCING CIRCUIT Robert L. Forgacs, Detroit, Mich, assignor to Ford Motor Company, Dearborn, Mich., a corporation of Delaware Application August 23, 1956, Serial No. 605,757

11 Claims. (Cl. 315241) This invention relates to a novel electrical circuit and physical arrangement of components for producing a short brief flash of light suitable for high speed photography. More specifically it deals with a circuit in which a switch is connected in such a manner as to initiate the discharge of a light gap at a desired instant.

Previously, it has been the usual practice to use'one of three systems, the first of which is to place a triggerable switch and a light gap in series across a capacitor which is to be discharged through the gap. In this type of circuit the series impedance of the discharge circuit is relatively high because of the added impedance of the series switch and connecting leads, resulting in a flash of extended duration and low circuit efficiency. Furthermore, in this circuit the ratio of power supply voltage to discharge capacitor voltage must be undesirably high.

The second known circuit employs a three-electrode light gap connected directly across a discharge capacitor, an ionization trigger pulse being employed to initiate discharging of the capacitor. It has been found that when an ionization trigger is used, a smaller light output is produced during the initial high voltage phase of the discharge than if breakdown were achieved by the sudden application of a high voltage pulse, Without the presence of any ionizing influence. The presence of the ionized path lowers the ratio of initial gap impedance to external circuit impedance. Furthermore, the presence of the third electrode in the light gap presents certain problems; if it is mounted to the side of the main gap, it may interfere with light emission, may make adjustment of the gap to small spacings difiicult, and is particularly subject to erosion which is more critical in a three electrode gap than in a two electrode gap. If it is'axially mounted in one' of the main'electrodes, fabrication is more difficult and the erosion problem still exists. Additional objections to this circuit are that one side of the gap is at a high voltage creating a safety hazard and also, the ratio of power supply voltage to discharge capacitor voltage, is undesirably high. H

A third type of firing circuit which has been employed utilizes two capacitors which are charged in parallel and discharged in series. The circuit arrangement is such that when a thyratron is triggered, one of the two capacitors discharges through the thyratron and an inductor, re-

versing its polarity. The series combination of capacitors then discharges through the light gap, which has a breakdown voltage considerably lower than the peak transient voltage; One disadvantage of this type of circuit is that when delay lines are employed as the discharge element,

the pulse width and impedance are both larger for the series combination than for a single line of half the capaci- 'tance and double the voltage rating of the individual lines.

Additional disadvantages are the possibility of damaging the thyratron and storage capacitor with the transient voltage reversal, and, the long recharging time precludes the possibility of having successive sparks in a short interval.

my invention to provide 2,931,948 Patented Apr. 5,

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a trigger circuit in which the efl'iciency of light output. is a maximum. Another object is to provide a circuit in which the duration of the light flash is very brief.

, Still another object is to provide a trigger circuit for a light gap in which both sides of the light gap are grounded except during discharge. l

Yet another object is to provide a circuit for triggering a light gap wherein the gap spacing may be easily adjusted. 1'

A further object is to provide a circuit for initiating a spark discharge across a light gap in which the power supply voltage requirement is a minimum.

A still further object is to provide a s tch gap and a firing circuit wherein the performance of the switch gap is reliable and efiicient over a wide range of operating conditions. a

An additional object is to provide a circuit in which breakdown of the light gap is automatically synchronized with and stabilized by the switch element.

These and many other objects and advantages of my invention will be appreciated upon a reading of the attendant specification and drawing which shows schematically the developed circuit.

Referring now to the drawing in detail; an electric power supply, designated as 11, provides the required high voltage for the circuit. This may be any suitable source which provides a voltage, for example, in the range'of l kv. to kv., the exact value depending on the capacity and voltage ratings of the circuit capacitors, which ratings are in turn a function of the light pulse intensity and duration requirements. The proper design values for the power supply and other components necessary to produce a pulse'of given intensity and duration can be determined by known methods of computing such values. A storage capacitor 12 is connected across the power supply through a charging resistor 13. Capacitor 12 is grounded at A. The ungrounded side of capacitor 12 is connected through a charging inductor 15 to one of the main electrodes 27 of a switch gap 16. To provide a more uniform field, the electrodes 27 and 28 of switch gap 16 are preferably made spherical or oval and have substantially equal diam? eters. Of course, the ratio of the diameters may vary over a wide range (in the limiting'case one electrode may be a plane, i.e., a sphere of infinite diameter). The other electrode 28 of switch gap 16 is connected to the ungrounded plate of a discharge capacitor 23. A bleeder resistor 22 is connected between electrode 28 and ground and'the second plate of discharge capacitor 23 is connected directly to ground. A light gap 24 is connected across the terminals of the discharge capacitor 23 and a trigger electrode 17 is incorporated in switch gap 16. This trigger electrode is connected to one terminal of an isolating gap 18 while the other terminal of gap 18 is connected through a current limiting resistor 19 to one side of the secondary of a pulse transformer 21 the'other side of which is grounded. The primary of transformer 21 is connected to a trigger generator 20 which is adapted to emit a high voltage pulse upon the happening of a selected external event.

In use my device operates in the following manner: the storage capacitor 12 is charged through the charging resistor 13 to the output voltage of the power supply 11'. When the proper signal arrives at the trigger generator 20, the transformer 21 emits a high voltage pulse which is applied to the trigger electrode 17 of the switch gap 16; isolating gap 18 breaking down in the process. The voltage on the trigger electrode rises until breakdown of the switch gap occurs from the negative electrode, to the trigger electrode, to the positive electrode. As the switch gap 16 breaks down, it emits ultraviolet light. The storage capacitor 12 then discharges through the charging inductor 15, and the switch gap, charging discharge capacitor 23 toward its peak transient voltage, isolating resistor 19 serving to limit the main discharge current passing through pulse transformer 21 during the transient. The ratio of peak discharge capacitor transient voltage to storage capacitor steady-state voltage approaches two as the ratio of storage capacitor capacitance to discharge capacitor capacitance approaches infinity. Therefore, by choosing the values of these capacitors properly, voltage multiplication can be obtained. Capacitor 23 charges to the breakdown voltageof light emitting gap 24, which is adjusted to break down at some voltage below the peak transient voltage. The spark which occurs at breakdown produces the desired flash of light. During the period between flashes, any excess charge leaks off through the bleeder resistor 22 thereby preventing the capacitor 23 from retaining an undesired potential.

It is desired to stress the physical arrangement of the components for it plays an important role in one of the features of this invention. The switch gap is arranged so that the'ultraviolet light from the switch gap follows an unobstructed optical path in its travel to the negative electrode '14 of the light emitting gap 24, consequently a portion of the ultraviolet radiation emitted from the switch gap 16 falls on this electrode. This ultraviolet illumination upon the negative electrode produces suflicient photoelectrons to stabilize the firing delay, and therefore the breakdown voltage of the light gap.

By combining the switch gap and the breakdown stabilizing elements into one unit, I am able to provide a 'simple and precise means of assuring that the stabilizing ultraviolet light will fall on the negative electrode 14 of the switch gap at the exact instant it is required. By thus synchronizing the ultraviolet radiation with the firing of gap 24, it is possible to adjust the gap to break down at a voltage much nearer the peak allowable capacitor voltage than when overvolting is employed to insure reliable operation. For a given discharge capacitor this means that the breakdown voltage of the gap may be increased, thereby boosting the light output of the flash. In addition, the intensity of individual light flashes, as well as the firing time delay, are both more consistent than when overvolting is employed.

The design of the switch gap per se constitutes another novel feature of the invention. It is well known in the art that the sparking potential of a gap is affected to a great degree by the uniformity of the inter-electrode field, which is in turn a function of gap geometry. When a third or trigger electrode is introduced into the field of a two-electrode gap, a certain amount of distortion is introduced. If the trigger electrode is located at one side of the gap, the required proximity of the trigger electrode tip to the gap axis depends upon the ratio of the peak trigger voltage to the gap operating voltage, as well as the absolute value of these voltages. When using a side-mounted trigger electrode and operating at high gap voltages (about 30 kv.) with trigger voltage peaks of about 25 kv., if the trigger electrode is positioned sufficiently close to the gap axis for reliable triggering, the resultant field distortion causes wide variations in the self breakdown voltage of the switch gap, making performance unreliable.

If the diameter of the smaller of the main electrodes of the switch gap is made large in comparison to the electrode spacing and the triggering electrode is made to float in the electrostatic field, the uniformity of the field is greatly improved. The floating of the electrode '17 is accomplished by providing the gap 18 to isolate the trigger electrode from the trigger voltage generator and by shaping the trigger electrode to lie approximately along an equipotential line in the region nearest the gap axis. If gap 18 is large enough to prevent breakdown when the trigger electrode rises to its floating potential and yet small enough to break down upon application of the trigger pulse, consistent operation over a wide operating range is obtained; Although this novel design of the 4 switch gap and its associated firing circuitry greatly improves the operation of the system, it is by no means limited to the circuitry set out hereinabove. It may be used in any circuit in which a triggering pulse is utilized to initiate transient operation of an electrical component.

It is also obvious that placing the ultraviolet-emitting switch gap adjacent to the component to be flashed so that the ultraviolet light produces photoelectric emission within the component will enhance the operation of any transient operated gap regardless of the particular circuit configuration involved.

From the foregoing, it is evident that this invention may have a number of equivalent embodiments. It is to be understood that my invention is not to be limited to the particular embodiment shown and described herein but may include any and all forms which are encompassed by the claims.

1. An electrical circuit comprising; a normally charged storage capacitor; means for charging said capacitor; :1 light producing gap having a predetermined breakdown voltage; a discharge capacitor connected in parallel with said gap and adapted to discharge through said gap upon reaching said predetermined voltage, a normally open, triggerable, switch connected in series between said storage capacitor and said discharge capacitor and having a trigger electrode, said discharge capacitor being electrically connectable to said storage capacitor through said' triggerable switch said tn'ggerable switch being adapted to close and thereby initiate charging of said discharge capacitor by said storage capacitor to a potential at least equal to the breakdown voltage of said gap upon application of a trigger pulse to the trigger electrode of said switch; means for applying a trigger pulse to the trigger electrode of said switch; and an inductor connected in series between said storage capacitor and said discharge capacitor to electrically isolate said capacitors from each other upon discharge of said discharge capacitor.

2. An electrical circuit comprising; a storage capacitor, power supply means charging said capacitor, a discharge capacitor, flash producing means operable by the discharge of said second named capacitor and connected in parallel therewith, said flash producing means having a photoelectric electrode, said discharge capacitor adapted to discharge through said flash producing means upon reaching a predetermined potential, a normally open, triggerable switch gap interposed between said capacitors, said discharge capacitor being electrically connectable to said storage capacitor through said switch gap said switch gap adapted to breakdown upon application of a trig'gerpulse thereby providing a closedjel'ectrical path between said capacitors so that said storage capacitor charges said second named capacitor to a potential at least equal to the potential required to initiate discharge of said discharge capacitor through said flash producing' means, the electrical" energy flowing across said switch gap producing radiation, said photoelectric electrode being positioned to receive said radiation, an inductor connected in series between said capacitors to isolate them'from each other upon the discharge of said second named capacitor, and means applying a trigger pulse to said switch.

3. An electrical circuit comprising; means supplying electrical energy, means operable by electrical energy, a switch gap serially connected between said supply means and'sa'id operable means said means operable by electrical energy and 'said means supplying electrical energy being electrically connected when said switch gap is closed, said switch gap comprising a positive electrode, a negative electrode and a floating trigger electrode shaped to lie substantially along an equipotential line of the electrostatic field between said positive and said negative electrodes, pulse generating means, an isolating gap connected in series-between said trigger electrode and said pulse generating means; said pulse means capable of generating a voltage pulse sufiicient to break down said isolating gap and thereby apply a voltageto said trigger electrode to close said switch gap. a 4. l'ncombination with a pulse generator; an improved switch gap adapted to be closed by a pulse'from said generator; said switch gap comprising; a generally spherical positive electrode, a generally spherical negative electrode spaced from said positive electrode at a distance not greater than the diameter of the smaller of said electrodes, and a trigger electrode, said trigger electrode shaped to lie substantially along an equipotential line in the electrostatic field between said positive and negative electrodes, an isolating gap having a breakdown voltage connected in series between said pulse generator and said trigger electrode so that said trigger electrode floats at a potential governed by the field between said positive and negative electrodes when said isolating gap is open, said isolating gap adapted to be closed when the voltage applied by said pulse generator reaches the breakdown voltage of said isolating gap thereby applying the pulse from said generator to said trigger electrode.

5. In combination with a pulse generator; an improved switch gap adapted to be closed by a pulse from said generator; said switch gap comprising; a positive electrode, a negative electrode spaced from said positive electrode and a trigger electrode, lying in the electrostatic field between said positive and negative electrodes, an isolating gap having a-breakdown voltage connected in series between said pulse generator and said trigger electrode so that said trigger electrode floats at a potential governed 'by the field between said positive and negative electrodes when said isolating gap is open, said isolating gap adapted to be closed when the voltage applied by said pulse generator reaches the breakdown voltage of said isolating gap thereby applying the pulse from said generator to said trigger electrode.

6. An electrical circuit comprising; a trigger pulser adapted to generate a voltage pulse upon the happening of a selected external event, means operable by a transient voltage, charged electrical storage means, a normally open switch gap said charged electrical storage means being electrically connectable across said means operable by a transient voltage through said switch gap, said switch gap comprising at least two main electrodes, a trigger electrode lying in the field between the main electrodes, and an isolating gap connected in series between said trigger pulser and said trigger electrode whereby the trigger electrode floats at a potential determined by the electrostatic field surrounding said trigger electrode, said trigger pulser intermittently impressing a voltage across said isolating gap suficient to break down said isolating gap and close said switch gap thereby providing a transient electrical path for said storage means to discharge through said operable means.

7. An electrical circuit comprising; means supplying electrical energy, flash producing means operable by electrical energy and having a photoelectric electrode, a switch having a plurality of main spaced electrodes to form a switch gap and a trigger electrode located adjacent said main electrodes, said switch gap connected in series with said supply means and said means flash producing said flash producing means being electrically connectable to said means supplying electrical energy through said switch gap, pulser means generating a voltage pulse upon the happening of a selected external event, an isolating gap connected in series between said trigger electrode and said pulser means, said isolating gap breaking down and applying a voltage to said trigger electrode suflicient to cause electrical energy to flow across said switch gap when said pulser'rneans generates a voltage pulse, the electrical energy flowing across said switch gap causing emission of actinic radiant energy, said photoelectric electrode of said flash producing means and said switch gap being so located that a portion of pacitor, means for charging the actinic radiant energy emitted by the electrical energy flowing across the switch gap fallsupon the photoelectric electrode of said flash producing means to stabilize the operation of said flash producing means in synchronism with the closing of said switch gap.

'. 8. In an electrical circuit, means for supplying'electrical energy, flash producing means operable by said electrical energy and having a photoelectric electrode, a switch gap positioned between said means for supplying electrical energy and said flash producing means, said flash producing means being electrically connectable with said means for supplying electrical energy through said switch gap said switch gap breaking down upon the application of a selected voltage thereby producing a flow of electrical energy across said switch gap, said flow of electrical energy causing radiant energy to be emitted from said switch gap, said photoelectric electrode being positioned to receive the radiant energy from said switch gap, the radiant energy falling upon said photoelectric electrode stabilizing the operation of said flash producing means in synchronism with the breaking down of said switch gap.

9. In an electrical flash producing system, a flash producing means having a photoelectric electrode, 2. capacitor connected across said flash producing means, a source of electrical energy for charging said capacitor, a switch gap positioned between said capacitor and said source of electrical energy, said capacitor being electrically connectable to said source of electrical energy through said switch gap said switch gap breaking down upon the application of a selected voltage thereby producing a flow of electrical energy across said switch gap, said flow of electrical energy producing radiant energy, producing radiant energy, said photoelectric electrode being positioned to receive radiation from said flow of electrical energy across said switch gap, said source of electrical energy charging said capacitor through said switch gap, said capacitor discharging through said flash producing means when the voltage across said capacitor reaches a selected value.

10. In an electrical flash producing system, a first casaid first capacitor, a second capacitor, a switch gap positioned between said first and said second capacitor, said second capacitor being electrically connectable across said first capacitor through said switch gap, means for applying a voltage to said switch gap of sufficient potential to break down said switch gap to cause electrical energy to flow across said gap and to cause the charging of said second capacitor, the electrical energy flowing across said switch gap causing radiant energy to be emitted therefrom a flash producing means connected across said second capacitor, said flash producing means having a photoelectric electrode positioned to receive radiation from the switch gap, said second capacitor discharging through said flash producing means when the voltage across said second capacitor reaches a selected level.

11. In an electrical flash producing system, a capacitor, a flash producing means connected across said capacitor, means for charging said capacitor, a switch gap positioned between said capacitor and the means for charging said capacitor, said capacitor being electrically connectable to said means for charging said capacitor through said switch gap means for applying a voltage to said switch gap of sufficient potential to break down said switch gap to cause a flow of electrical energy across said gap and to cause the charging of said capacitor from said means .for charging said capacitor, the electrical energy flowing across said switch gap causing radiant energy to be emitted therefrom, said flash producing means having a photoelectric electrode positioned to receive radiation from the switch gap, the radiation from said switch gap falling on the photoelectric electrode of said flash producing means to produce electrons on the surface thereof whereby said capacitor discharges through said flash 

